This invention relates to sol-gel processes for making large sol-gel bodies. It is especially applicable to techniques for preparing optical fiber preforms prior to fiber draw.
A variety of methods have been suggested for the manufacture of high-silica content glass articles, such as the single and double dispersion processes described by D. W. Johnson, et al. in Fabrication Of Sintered High-Silica Glasses, U.S. Pat. No. 4,419,115, and the process described by D. W. Johnson, et al in Sintered High-Silica Glass And Articles Comprising Same, U.S. Pat. No. 4,605,428. Uses of high-silica content include the fabrication of glass rods for use as preforms in the manufacture of optical fibers as suggested by F. Kirkbir, et alii, U.S. Pat. No. 5,254,508 for a Sol-gel Process For Forming A Germania-doped Silica Glass Rod, and the fabrication of secondary cladding tubes for use during fabrication of an optical fiber by a solgel process. Although sol-gel processes enable fabrication of glass objects at lower cost than other processes, N. Matsuo, et alii, in U.S. Pat. No. 4,680,046 for a Method Of Preparing Preforms For Optical Fibers, among others, has noted that it is difficult to provide a glass article that is large enough to be used as a preform for optical fibers.
Considering that the functioning part of an optical fiber (the core and inner cladding carrying 99+% of the optical energy) typically consists of but 5% of the mass, a significant part of this effort has concerned structures providing for overcladding of such inner portion. State of the art manufacture often makes use of an inner portion constituting core and inner clad region as fabricated by Modified Chemical Vapor Deposition, or, alternatively, by soot deposition in Outside Vapor Deposition or Vapor Axial Deposition. This core rod may be overclad by material of less demanding properties, and, consequently, may be produced by less costly processing. Overcladding may entail direct deposition on the core rod, or may result from collapsing an encircling tube. Such xe2x80x9covercladdingxe2x80x9d tubes have been produced from soot or fused quartz. Making very large bodies of soot require extensive processing, and large bodies of fused quartz are expensive.
It has been recognized that significant economies may be realized by fabricating overcladding tubes by sol-gel techniques. This well-known procedure is described, for example, in J. Zarzycki, xe2x80x9cThe Gel-Glass Processxe2x80x9d, pp. 203-31 in Glass: Current Issues, A. F. Wright and J. Dupois, eds., Martinus Nijoff, Boston, Mass. (1985). Sol-gel techniques are regarded as potentially less costly than other known preform fabrication procedures. While sol-gel fabrication of overcladding tubes, and other optical glass components, has met with considerable success, improvements are continually sought.
A persistent problem in making very large sol-gel bodies, e.g. greater than 5 Kg, for state of the art optical fiber drawing is cracking of the gelled body. Cracking may occur during drying or handling of the gelled body prior to consolidation. See for example, T. Mori, et al, xe2x80x9cSilica Glass Tubes By New Sol-Gel Methodxe2x80x9d, J. Non-Crystalline Solids, 100, pp. 523-525 (1988), who describe the cracking problem, and recommend modification of the starting mixture and of the gel forming process, both of which are involved and expensive. The cracking problem is explained in a paper by Katagiri and Maekawa, J. Non-Crystalline Solids, 134, pp. 183-90, (1991) which states, xe2x80x9cOne of the most important problems in the sol-gel preparation method for monolithic gels is avoidance of crack formation which occurs during dryingxe2x80x9d. A 1992 paper published in the Journal of Material Science, vol. 27, pp. 520-526 (1992) is even more explicit: xe2x80x9cAlthough the sol-gel method is very attractive, many problems still exist, as pointed out in Zarzycki. Of these problems, the most serious one is thought to be the occurrence of cracks during drying of monolithic gelxe2x80x9d. The reference then reviews remedies, e.g. hypercritical drying procedures and use of chemical additives such as N,N dimethylformamide, collectively referred to as Drying Control Chemical Additives. Both methods are regarded as expensive and, therefore, undesirable in routine glass production. An extensive description of a suitable sol-gel process, and of additives useful for improving the strength of sol-gel bodies, is contained in U.S. Pat. No. 5,240,488, which is incorporated herein in its entirety.
The cracking problem becomes more severe as the size of preforms in commercial fiber production increases. State of the art optical fiber manufacture typically involves drawing hundreds of kilometers of fiber from a single preform. These preforms typically exceed 5 Kg in size. Although improvements in techniques for making large sol-gel bodies have been made, strength continues to be an issue and any process modification that results in improvement in the strength of intermediate products during the sol-gel process will constitute a valuable contribution to the technology.
We have developed a modified colloidal sol-gel process for making large sol-gel bodies of silica, and silica-containing, glasses. The modification takes advantage of a surprising discovery that the starting material in the sol-gel process, silica particulates, may be much larger than previously thought. We formulated sol-gel bodies using colloidal suspensions of silica particles in the size range defined by 5-25 m2 per gram. These particles are substantially larger than those typically recommended, i.e. 50 m2 per gram. Contrary to expectation, colloids formed with these large particles did not result in premature settling, as would have been expected.
Also contrary to expectation, wet sol bodies with very high solids loading, i.e. 65-78%, may be obtained using large particulate starting materials and proper processing. These large loading quantities are found to improve wet strength without impairing sol stability and rheology. Due to the large loading, shaped sol bodies in the xe2x80x9cgreenxe2x80x9d state more closely match the dimensions of the final desired form and thus allow for more complex shapes and greater dimensional control.
It was also found that particle morphology contributes significantly to the improved results. Particles with essentially spherical shapes are necessary for the results obtained. Conventional silica particle mixtures contain both spherical and non-spherical particles, the latter in quantities of 30% or more. We have achieved the improved results reported here using particle mixtures with less than 15% and preferably less than 10% non-spherical.
Sol-gel bodies formulated using these starting colloids result in strength improvements of 100%, and in some cases, 300%. The enhanced strength as well as higher loading allows faster drying of the sol, thus reducing overall processing time. The reduced surface area per gram also allows additives to be included in smaller amounts. This lowers the cost of materials and also decreases the process time required for burn-off of additives. These processing efficiencies translate into lower production cost, especially for very large bodies.